666 MAMMALIA. 



poises (Delphinidte), and of tlie Caclielot Whales (Physeler). The 

 example selected to illustrate their structure and mode of growth is 

 a preparation of a portion of the jaw of the Bottle-nose Whale 

 (Delphinus Tursio) contained in the Hunterian collection.* 

 From this it is seen (Jig. 311) that each tooth of the Cetaceans in 

 question is a hollow cone of ivory (a, >, c, d), which, on being split 

 longitudinally, is found to contain a vascular pulp, exactly filling 

 up its internal cavity. Tt is upon the surface of this pulp that the 

 ivory matter is produced and deposited, stratum inter stratum, 

 within the tooth, thus gradually adding to its substance as growth 

 proceeds. In animals possessing a dental apparatus of this descrip- 

 tion, Mr. Hunter observed that the teeth are not at first developed 

 in the jaw, but appear to form in the gum upon the edge of the 

 maxillary bones ; and that they either sink into the jaw as they 

 lengthen, or, as is more probably the case, the alveoli rise to en- 

 close their roots as growth advances. It would moreover appear 

 that these creatures do not shed their teeth ; but that, as the jaw 

 enlarges, new teeth are constantly produced from behind, while 

 those towards the symphysis fall off, and their sockets become 

 absorbed : thus the size of the teeth is made to keep pace with the 

 increasing dimensions of the jaw.-)- The exact number of teeth 

 met with in any species of these Whales will evidently be un- 

 certain. 



In the male Narwal (Monodon) there are no teeth implanted 

 along the margins of the jaws ; but from the intermaxillary bone of 

 the left side of the face there projects a single tusk of great strength, 

 which sometimes attains the length of eight or ten feet. This 

 formidable weapon is fully developed only upon one side of the 

 body ; nevertheless, the corresponding tooth exists in a rudimentary 

 condition, enclosed in the opposite intermaxillary bone. 



In the Elephant, a creature which so obviously forms a connect- 

 ing link between the gigantic Cetacea and terrestrial quadrupeds, 

 tusks, more ponderous even than that of the Narwal, project 

 from both intermaxillary bones : but these, as well as the tusks 

 of other PACHYDERM AT A, grow upon a simple pulp, such as that 

 which forms the teeth of the Bottle-nose Whale; are formed of 

 ivory, without any enamel ; and their growth is only limited by the 

 abrasion to which they are subject. 



* Preps. No. 327 and 328. 



t The Animal CEconomy, by John Hunter, with notes by Richard Owen, Esq. F.R.S. 

 p. 353. London, 1837. 



